Background
The authors write that "understanding the long-term natural variability of tropical cyclones (TCs) is important for forecasting their future behavior and for the detection and attribution of changes in their activity as a consequence of anthropogenically induced climate change." And they state, in this regard, that "critical to these endeavors is determining whether, over the long-term, TCs occur randomly or display identifiable patterns influenced by one or several factors."

What was done
In an effort designed to shed some light on this important subject, Nott and Forsyth present what they describe as "new sedimentary data from the southwest (SW) Pacific and southeast (SE) Indian Ocean regions which allow us to make comparisons with existing sediment records from the Atlantic Ocean (Donnelly and Woodruff, 2007; Mann et al., 2009), northwest (NW) Pacific (Woodruff et al., 2009), Gulf of Mexico (Liu and Fearn, 1993, 2000; Lane et al., 2011) and the Gulf of Carpentaria, Australia (Rhodes et al., 1980)."

What was learned
The two Australian researchers report that "long-term global TC activity is not random." Rather, there is, as they demonstrate, "a substantial degree of synchroneity in global intense TC behavior over the past 3,000 to 5,000 years." Indeed, they say that "one of the most striking aspects of these records is they all display extended alternating periods (centuries to millennia) of relative quiescence and heightened intense TC activity irrespective of both the resolution and type of long-term TC record."

What it means
Something yet unknown has orchestrated the ebbing and flowing of global TC activity over the last 5,000 years. What we do know, however, is that it has not been changes in the atmosphere's CO2 concentration, which has remained quite stable over this entire period ... except for the past 100 years, when it has risen substantially, but without any demonstrable change in global TC activity. Hence, there is no compelling reason to believe that any further increase in the air's CO2 content will have any significant impact on these destructive storms.

References
Donnelly, J.P. and Woodruff, J.S. 2007. Intense hurricane activity over the past 5,000 years controlled by El Niņo and the West African monsoon. Nature447: 465-468.